Sept., 1962
ORTHO-PARA-~~YDROGEN CONVERSION BY METALSURFACES
1715
ORTHO-PARA-HYDROGEN COXVERSION BY METAL SURFACES AT 81O K . BY J. T. KUMMER Scientific Laboratory, Ford Motor Company, Dearborn, Michigan Received April io, 1968
Ortho- ara hydrogen conversion has been used at 21'K. t o study the magnetic roperties of Cu, Ni, Fe, Au, Cu-Ni, and FeAu sur!aceE, and oxidized Ni and Fe surfaces. Copper and gold surfaces exhigit a slight paramagnetism, the origin of which is not known. It can be described in terms of an equivalent surface containing two unpaired electrons per 100 surface atoms in the case of copper and one unpaired electron per 100 surface atoms in the case of gold. The surfaces of nickel and copper-nickel alloys show a surface paramagnetism which is the same as that found for copper. No evidence was found for localized moments on the surface of the highest copper-nickel alloy used (-6% Ni) or on nickel itself other than the description given above for copper. Iron surfaces exhibit strong localiaed moments as do iron atoms in a gold-iron alloy surface. Oxidation produces strong moments on the surfaces of both nickel and iron.
Introduction The conversion of ortho to para-hydrogen is generally recognized to occur by two different mechanisms. The first involves dissociation of hydrogen molecules and recombination of the atoms. The second is purely physical in nature and is induced by an inhomogeneous magnetic field such as exists as a result of the paramagnetism of unpaired electrons, or of nuclear moments. The magnetic conversion in a homogeneous phase is understood, as a result of Wigner's' work, in terms of the time spent within the perturbing field. The heterogeneous conversion on surfaces has been observed in a qualitative way by many workers, but rarely under conditions permitting quantitative interpretation and where a clear distinction is possible between the magnetic and dissociative mechanism. I n recent, years the work of Sandler,2 Harrison and M ~ D o w e l l ,and ~ Cunningham and Johnston4 has made substantial progress in the understanding of the kinetics of the magnetic mechanism on surfaces. As a consequence, the study of the ortho-para-hydrogen conversion induced by perturbing inhomogeneous magnetic fields present in solid surfaces is a potentially powerful tool for improving our understanding of the magnetic properties and composition of such surfaces. l'articularly important is the conversion a t liquid hydrogen temperatures where the possible interference of the dissociative mechanism is virtually non-existent, and the over-all kinetics are simple by virtue of the presence of a complete monolayer of nearly 100% ortho-hydrogen on the surface. The present work was undertaken to extend the application of the magnetic ortho-para-hydrogen conversion a t liquid hydrogen temperatures to the study of metal-alloy surfaces. Following the line of reasoning put forth by Harrison and M ~ D o w e l lone , ~ can visualize a surface as composed of c paramagnetic sites per cm.2 of magnetic moment ps. When this surface is covered by adsorbed hydrogen, the rate of para-hydrogen production in cc. S.T.P./min./cm.2 of surface is given by the expression (1) E. Wigner, 2. physik Chem., B23, 28 (1933). (2) Y. L. Sandler, J . Chem. Phys., 20, 1050 (1952); ibid., 21, 2243 (1953); Can. J . Chem., 3 3 , 2 4 9 (1954); J . Phys. Chem., 58, 54 (1954); abad., 68, 1101 (1959). (3) L. G. Harrison and C. A. McDowell, Proc. Roy. Soc. (London),
A220, 77 (19.531. (4) C . M. Cunningham and H. L. Johnston, J . Ana. Chem. Soc., 80, 2377 (1958).
where Kt2/r8 is the Wigner probability that an ortho-hydrogen molecule will be converted to parahydrogen as it passes over a paramagnetic site. T is the distance of the molecule from the site paramagnetic moment and t is the time of interaction with the site. n is the number of hydrogen molecules passing over each site per unit time and c is the number of sites per cm.2 of surface. n can be expressed as n*a where n* is the number of ortho molecules passing over each per unit time and a is the site area. Again following Harrison and McDowel13we can write t = b/u
(2) where b is the root mean square of all the paths through the paramagnetic site area, and v is the velocity of lateral motion of the adsorbed hydrogen molecules along the surface. Using Wigner's expression for K
K
=
A,,?
(3)
where A is a constant and pCsis the effective magnetic moment of the paramagnetic site in Bohr magnetons, one can write for the rate
(4)
If one works a t one temperature only and under conditions where the surfaces are covered with a t least one monolayer of adsorbed hydrogen, then n* and v in the expression will be constant. If the site has a diameter d and if this d changes as it will when one measures the conversion rate produced by different metals, then b2a will vary as d4. If one-half of r varies as d(r = radius of hydrogen radius of site) then r* also varies as molecule d4 for small variations in d. Because of this and because of the difficulty in estimating b, a, and r , we have assumed that part of (4) in the brackets to be constant a t any one temperature, and have used the expression
+
R = Bps2c (5) for interpretation of our results. We have worked exclusively a t 21.5' (vapor press. of H2 = 1000 mm.) and a t hydrogen pressures over our sample
1716
J, T. I ~ M I V I E R
of from 0.3 to 0.7 the vapor pressure at 21.5OK. SO that the surface will be covered5 with a t least one monolayer of hydrogen. The hydrogens in the second layer are not converted due to the large dependence on distance. The rate has been found to be independent of total pressure in this range. It has been s h o ~ n that ~ ~ ortho-hydrogen ~ - ~ is preferentially adsorbed over para-hydrogen, so that during an experimental ortho-para conversion run where the gas phase ortho-hydrogen may vary from 75% to as low as 30%, the surface remains very nearly completely covered with ortho-hydrogen so that the rate of para production, R, is constant with time. Since the equilibrium per cent para is -99.7% a t 21.5'K., one can neglect the back reaction in the range 75 to 30% ortho. This means that if one measures the total cumulative cc. S.T.P. of para made in an experiment in a given time, as the gas phase ortho concentration varies from 75 to -30%, the rate of para production R that we want to use in eq. 5 is just the cc. of para made divided by the time of the experiment. Although the constant B in eq. 5 can in principle be calculated, we have used the data of Johnston and Cunningham4 for the conversjon of ortho to para-hydrogen a t liquid hydrogen temperature over surfaces with known Cr+3content for this purpose. From their data for 20 Cr-A1 in which they have 231 X 10-5 g. atom of Cr+3on the surface and an initial rate of 1960 cc. S.T.P. of paraproduced/min., one can calculate using p = 3.87 Bohr magnetons that B = 0.94 X lO-lY. With this value of B in eq. 5 one can find p 2 c for a metal surface if one has measured R, the rate of ortho-para-hydrogen conversion over the surface a t liquid hydrogen temperatures. If there is available additional information on either p or e, the other can be obtained. If no additional information is available, one can characterize the surface in terms of an equivalent one in which p = 1.73 and express it in terms of so many unpaired or per 100 surface atoms. electrons per Shallcross and Russellg have measured the ortho-para-hydrogen con\-ersion rate over similar samples of Cu and Ni and alloys a t 77OK. and above and report, activation energies of 6-15 cal./mole for the reaction (chemical mechanism). Their rate data extrapolated to 21'K. give a value much too low to be measured, so that a t 21°K. we are measuring a rate due to the magnetic mechanism only. Experimental The hydrogen used was freed of oxygen by hot cop er (360") and passed through a charcoal trap at 77°K. in orxer to remove any nitrogen that may have been present. The gold was made by reduction of gold chloride in acid solution by hydrazine. The iron-gold alloy was made by melting the two metals together in silica in a hydrogen atmosphere followed by quenching. After annealing, the X-ray diffraction 1in:s were very sharp, giving a lattice parameter of 4.064 A . for the 2 iron in gold alloy. a T t . ~ o
( 5 ) 4 . Van Itterbeek and ,J. Borghs, 2. physzb. Chem., B60, 128
(1941). (6) Y. L. Sandler. J . Phus Chem., 68, 58 (1954). (7) D. White and E. N Lassettre, J . Chem. Phyr., 32. 7 2 (1960). (8) E. Cremer, 2, p h y a k Chem., B49, 245 (1941); BZS, 383 (1935). (9) P. E. Shatlcrom @ndR. T1'. Russell, .b. Am. ChemmSoc,, 81, 4132 tle&B),
Vol. 66
This sample was filed up into -50 U, particles and boiled with concentrated nitric acid to remove any contamination from the file. Although this treatment removes iron atoms from the gold-iron alloy surface, the surface again becomes populated with iron atoms after 16 hr. reduction a t 530°, since under these conditions the surface and bulk are in diffusional equilibrium. The copper and nickel samples were reduced from the oxides. The copper oxide (Johnson Matthey) contained -20 p.p.m. metallic impurities, principally silicon, iron, and calcium. The coppernickel alloys were made by reduction of the co-precipitated carbonates. There is good evidence from the literature10 that reduction of the co-precipitated carbonates gives a homogeneous alloy. The metals, after hydrogen treatment as specified in the tables, were treated with a stream of helium purified by charcoal at 77°K. flowing at 100 cc./min. for 0.5 hr. a t reduction temperatures in order to desorb hydrogen from the surface, and the samples then were cooled in helium to 21.5'K. This temperature was maintained by the presence of liquid hydrogen in equilibrium with 1000 mm. of hydrogen pressure on the outside of the sample bulb (-7 cc. volume). The helium was evacuated a t 21.5"K. and hydrogen was added to 740 mm. a t the start of a run. Small samples were removed after intervals of time and analyzed for para-hydrogen by thermal conductivity. The plot of cumulative cc. of para made against time is a straight line for the first 50% conversion, the slope of which is the desired rate. The surface areas of the samples wefe measured by krypton adsorption at -195" using 20.8 A.2 per Kr atom. The sample was protected from stopcock grease by liquid nitrogen traps and the Apieeon grease used was mixed with colloidal silver in an effort to remove sulfur compounds.
Results and Discussion Figure 1 shows plots of some of the data. AS can be seen from the graph, the rate is zero order in ortho-hydrogen as expe~ted4~6,~ since 25 cc. of para-hydrogen represents progress about one-half the way to equilibrium. Table I summarizes the data for copper and copper-nickel alloys, Table I1 contains the data obtained for nickel, Table 111 contains data for iron, and Table IV for an irongold alloy. TABLE I COPPER-NICKEL DATA Sample
cu -4bove sample H? at 25", 41 hr. Cu, CO pre-treated a t 25' to remove Fe Ni Cu 0.64% Xi
+
+ + Cu + 1.26% Ni
Cu
+ 6 ~ 4 %Ni
Conversion rate R , CC. of para/min./cm.Z
Redn. temp., '(2.
Surface area, m.z/g.
360
0.19
~.
.19
,0065 X
300 270 430 360 430 550
.19 .72 .19 .27
,091 x ,108 X ,096 x ,102 x ,093 x .os7 x ,085 x
450
.I8
.16 .28
0,080 X
10-4
10-4 10-4 10-4 10-4
10-4
It has been known since the work of Bonhoefferll that the surfaces of diamagnetic solids can convert ortho to para-hydrogen, It has been proposed by Couper and eo-workers12 that this may be due to a surface lattice of unpaired electrons produced as a result of the projection of a free valency into space. Our results for copper show a conversion rate (10) W. K. Hall and L. Alexander, J . P h p . Chem., 61, 242 (1957). (11) K. F. Bonhoeffer, A. Farkas, and K. M'. R u m m e l 2. p h y s i k . Chem., B Z i , 225 (1933). (12) A. Couper, D. D. Eley, M.J. Hulatt, and D. K Rominpton, Bull. aoc. ehim, Balnes. 61, 34.1 61968).
ORTHO--PARA-HYDROGEN COXVERSION BY n T m x SURFACES
Sept., 1962
TABLEI1 NICKELDATA Reduc-
tion temp., "C.
Sampk
Surface area, m %/g.
d Area/
Ni A.2
+
1.26% NI 430T
2 u.
0
Lc
5
20
w
2
5 g
64%Nl430'C 10
I V J
z
_r__C-.-.C-Cu+H2
* 0
200
100
Rate, cc. of para produced/min./ pelf om 2 calod.
Johnson Matthey iron sheet dissolved and reprecipitated Reduced 490°, 16 hr. 7300 cm "g 5 5X 2 0 Above O2 a t 25' 7300 cm.2/g 19 X 4 3 Johnson Matthey sponge Reduced 16 hr., 490' 2100 cm.2/g 6 5 X lo-" 2 2 Above oxygen 2100 cm 2/g 16 X 10-J 3 9 Reduced 16 h r , 490' 1900 cm.2/g 5 9 X 2.1 Cooledin hydrogen 1900cm.2/g. 2 6 X 14 For J. h1. Sponge CO chemisorption gives 13.6 A . 2 per atom,(one C'O molecule per S i atom), HZchemisorption gives 13.3 A.2 per atom (one H atom per Ni atom).
+ +
R of -0.09 X lo4 cc. of para produced per minute per cm.2 of surface when the surface is completely covered with ortho-hydrogen. This would give p2c of 9.6 ;< 1013and if we assume sites of a moment of one unpaired electron, then there would be 3.2 X l O P of these paramagnetic sites. If we assume an average area of a copper atom of 7 A.2, then there are 1.4 X 1015atoms/cm.2, or one copper atom in 50 would have a moment of one unpaired electron. Our results also could be interpreted as each copper atom having a moment of 0.26 OS a Bohr mag,neton, if this were possible, but not by nuclear moments of the surface copper atoms or of the ortho-hydrogen (homogeneous liquid phase conversion), since these would be too small. Exposure of Ihe copper to hydrogen a t room temperature for 41 hr. resulted in a rate of conversion of l/l0 that for the sample as normally prepared by helium treatment a t reduction temperatures. Apparently, the sites that cause ortho-para-hydrogen conversion can also chemisorb hydrogen, The activation energy for this is high enough (KwanI3)so that this does not take place a t 21 OK. The nature of the paramagnetic site on the copper surface that is responsible for the conversion is not known. I[t may be of interest, to note that the lowest value of rate we found on our hydrogen poisoned copper is similar to that found by Cunf a p a n , '2.8, 7a (1960).
300
400
TIME, MINUTES,
Fig, 1.-Ortho-para-hydrogen conversions over coppernickel samples. The ordinate gives the total cumulative amount of para-hydrogen made for samples of varying Ha = 1.95 g., 1.267, Ni == 4.90 g., weights. Ni and Xi 0.64% Ni = 1.80 g., Cu 1%= 5.56 g. The temperatures given are the reduction temperatures.
+
TABLEI11
IRON DATA
T.Kfnan, Bull C h e m
30 P a
U
+
(13)
I.ZG%Ni 360T
3 0
Converslon rate R,cc. of para/min./cm.2 of surface
Baker'sanal.yzedn'i 360 1 60 0 096 X lou4 11 6 Above sample Hz .. 1 60 050 x at 25' Ni spec. pure 480 0 14 21 X 550 12 32 x 10-4 Further reduction Above sample Hz a t 25" 12 23 x 10-4 Further reduction 500 12 41 X 14 1 Above sample after\ 360 12 16 X 11 9 diluteHN0awashj 570 10 26 X Xi, oxide covered 14 2 23 X l o w 4 aFrom CO chemisorption at -78" assuming 1 CO/nickel atom.
Surface area
1717
+
ningham and Johnston for pure r-A1203,which converts 0.0052 X cc. of para/min./cm.2 of surface. The conversion rates over the Cu-Si alloys per unit area are not much different from the value for pure copper. If each surface nickel atom had a localized moment of one unpaired electron and if the surface concentration of nickel were the same as the bulk composition and the nickel existed as an isolated atom in the surface (without pairing), then one would expect for the 6% nickel alloy a rate per unit area three times that for the copper (the copper rate corresponded to two unpaired electrons per 100 surface atoms). Since this is not observed, we have concluded that, within the limitations given above, the surface nickel atoms do not possess localized moments. We have assumed that a surface nickel atom in a copper matrix does not chemisorb hydrogen a t 21°K. This will be discussed below with the nickel results. We were interested in seeing what we might find out about the magnetic inhomogeneity of a nickel surface using ortho-para-hydrogen conversion. Our principal concern was the possibility of hydrogen chemisorption a t 21OK. Although Beeck14 has shown that evaporated nickel films chemisorb hydrogen a t 20'K. and above, other experience has shown that hydrogen reduced powders chemisorb hydrogen slowly a t 77°K. Schuit and deBoerI5 have shown that the amount of hydrogen chemisorbed on nickel a t 77OK. is a function of the temperature of reduction, being low for reduction a t 300" and approaching total coverage after 500" reduction. Accordingly, we have measured the ortho-para conversion rate over nickel after various reduction temperatures in order to see if there were any evidence for a lowering of the rate after high temperature reduction due t o hydrogen chemisorption. Kone was found, so we concluded that hydrogen does not chemisorb on (14) 0. Beeck, J. W. Givens. and A. W. Ritchie, J. Colloid Sei., 6 , 141 (1950). (15) G. C. A. Schuit and N, H. deBoer, Rsc. Iran. :ham., 70, 1067 (1951).
1718
J. T. KUMMER
our reduced nickel a t 21'K. The effect of a small amount of oxygen left on the surface would be, as shown below, to increase the rate. AS can be seen from Table 11, the nickel made by reducing Baker's analyzed X i 0 gare a rate similar to copper, indicating little magnetic inhomogeneity of the surface. The rate over nickel made by reducing spec. pure NiO to a very smadl surface area material increased somewhat with increasing reduction time, which leads us to suspect that some paramagnetic impurity insoluble in the nickel was accumulating on the very small surface of this sample (such as Xis). Khen this sample was washed with 0.01 N H S 0 3 in conductivihy water and then re-reduced, the rate mas lower, which would be expected from removal of surface impurities (see Table 11). CO chemisorption also suggests removal of surface impurity. If each nickel atom possesses a moment of 0.6 unpaired electron or -1 effective Bohr magneton as judged from the saturation magnetic moment a t 20°K., one mould expect a rate of -1 X cc./min./cm.2, which is 10 times the rate found for the high area nickel. The effect of chemisorbed hydrogen is very much less in the case of nickel than in the case of copper. An oxide was formed on the nickel surface by exposure to oxygen a t room temperature followed by heat treatment in vacuo a t 200' for 15 min. in order to remove any remaining oxygen. As expected (Table 11), the conversion rate over this sample was very fast due to the presence of localized moments of the NiC2 ions. After the heat treatment, the surface probably was covered with patches of X i 0 and patches of oxygen chemisorbed on the nickel surface. If all the nickel were divalent, we would expect a rate of -7.5 X lo-* cc. S.T,P./min. from eq. 1, instead of the value 2.23 as found. The difference either reflects the inaccuracy of eq. 1or our method of measuring surface area, or that the nickel atoms on the surface under the chemisorbed oxygen have a 1.1 less than 2.83. The conversion rate over iron is very fast. I n fact, one could easily work with single crystals of iron of -10 cm.2 surface area, particularly if one worked a t p / p o of 0.3 and a t a lower temperature so as to lower the amount of gas in the sample bulb. If one assumes a value of 1.4 X 1015 iron atoms/ cm.2 of surface, which would seem reasonable, one can calculate an effective moment for each iron atom of 2-2.2 Bohr magnetons. This is lower than the effective moment of bulk iron of 3.1 magnetons. If, however, one uses the concentration of iron atoms on the surface as measured by CO and H2 chemisorptionle of 7.5 X 1014/cm.2,one obtains a pep of -2.8 for each iron atom, which is in better agreement with the bulk value. For the oxidized sample if one uses ferric ions per cm.2, which seems reasonable, one calculates a moment of -4 Bohr magnetons, which again is smaller than the value of 5.90 for ferric ion. The lowering of the rate due to chemisorbed hydrogen may be a result of the involvement of the unpaired electrons in the chemisorbed bond or it may be a steric effect. (16) Assuming one CO molecule per nickel atom or one hydrogen a t o m per nickel atom.
Yol. 66
In general, however, one can say that iron givcs high conversion rates in approximate agreement with eq. 5 and that failure to find agreement in the case of nickel would indicate that the nickel surfacc does not possess localized moments. The data for gold (Table IV) show that thp surface paramagnetism per unit area of the high area. gold is considerably higher than for the low area gold. It is not known whether this is due to the physical structure of the surface (a large number of edge and corner atoms) or whether this is due to some surface impurity. If it were due to an impurity, then this impurity must have been removed or altered by hydrogen reduction a t 500' since this treatment resulted in a much lower surface paramagnetism per unit area. This latter surface can be described as in the case of copper as an equivalent surface of one unpaired electron per 100 gold surface atoms. TABLE IS' Au, Fe-Au DAT.4 Area
Rate: cc. of para/niin./om.?
High area gold reduced 1 hr., 0 . 5 2 x 10-4 200 O 6800 cm:/g. Gold sample above reduced 2 hr., 500" 230 cm.2/g. .OM x 10-4 6 . 3 atom yo Fe-Au reduced .59 x 10-4 530°, 16 hr. 110 .44 x 10-4 Reduced CO - 78.5' 110 .50 x 10-4 Reduced HP - 7 8 . 5 " 110 Reduced 02 at 25' 110 3 . 8 x 10-4
+ + +
The iron-gold alloy after 530' reduction shows a high ortho-para conversion rate as compared to pure gold after reduction a t a similar temperature. If one uses 3.1 Bohr magnetons for the effective moment of the iron atom on the gold surface, then using eq. 5 one finds that there are 6 X iron atoms/cm.2. If one assumes 9 A.2 per gold atom on the surface, then there are 1.1 X 10l6 gold atoms/cm.2 and the atom % iron on the surface is 5.5 atom %. This is in favorable agreement with the bulk composition of 6.3 atom % iron. By carbon monoxide chemisorption, it was found that the atom % iron o,n the surface was 6.5 f 0.5%, again assuming 9 A.2per gold atom. The effect of CO and H2chemisorption was much less than expected since i t was thought that both of these would involve bonds using the unpaired electrons responsible for the magnetic moment of' the iron atom. The gold-iron alloy after reduction and treatment with helium was exposed to oxygen a t 25' for -1 hr. and then the oxygen was evacuated. The conversion rate over this surface increased as for pure iron. In the case of iron, one undoubtedly forms an iron oxide lattice perhaps 20 A.thick by 0 2 treatment a t room temperatures, the surface of which would contain the required ferric ions for conversion. In the iron-gold alloy case, the structure of the final product is not known, but from the rate observed one can say that the ferric ion is
KINETICSOF CHLORINE EXCIIAXGE BETWEEN CHLORIDE AND CHLOROACETATE IONS 1719
Sept., 1962
not s tericdly hindered from interaction with the hydrogen molecules any more than in the case of iron oxide itself. In sumniary, one can say that for the diamagnetic metals copper and gold one observes a small surface paramagnetism of unknown origin. For the transition metals, nickel and iron, one finds the existence of strong moments a t each surface iron atom butJno evidence for any at each surface nickel atom. Upon oxidation, cach surface nickel or iron atom under-
goes an increase in magnetic moment, For the binary alloy copper-nickel, there is no evidence for local moments at the surface nickel atoms, whereas for the binary alloy iron-gold, the experiment indicates that each iron atom has a moment of ~3 Bohr magnetic units. This method of measuring surface para,magnetism can be used to measure the surface composition of those binary metal alloys where one of the constituents possesses an isolated magnetic moment and the other does not.
KINETICS OF CHLORISE EXCHANGE BETWEEN CHLORIDE AND CHLOROACETATE IONSL BY F. J. JOHNSTON Department of Chemistry, University of Georgia, Athens, Georgia Received April 10,1968
I n aqueous solutions containing chloride and chloroacetate ions, exchange of chlorine between the two species occurs simultaneoiisly with hydrolysis of the latter. I n acetate buflered systems, the exchange rate is first order with respect to each of the chloride and chloroacetate concentrations and may be described by the expression R,(t) = 2.26 X 10" exp [( -26,400 & 400)/RT](Cl-)(CH~ClCOO-) moles I.-' see.-'. The corresponding entropy of activation evaluated for 80" is -8.9 cal. mole-' deg.-l. The simultaneous hydrolysis reaction was described by a pseudo-first-order behavior with the rate given by the expression Rh(t) = 3.83 X 1O1l exp[( -27,900 i 600)/RT] (CH&lCOO-) moles I.-' sec.-l.
[m
Introduction d (MCA-) * = R,(t) I n aqueous solutions a t temperatures above 70°, dt (Cl-) (MCA-) chloroacetate ion undergoes hydrolysis at a measurable rate with the production of chloride and glycolate ions. With chloride labeled with chlorine-36 present in the system, it is observed that exchange with chloroacetate occurs under virtually the same conditions as the hydrolysis. This article reports the results of a kinetics study of the simultaneous exchange and hydrolysis reactions. The terminology and equations used Ft is the ratio of specific activity of chloroacetate are discussed below. ion at time t to that of total chlorine in the system For the hydrolysis reaction a t exchange equilibrium. a and b refer to initial concentrations, (MCA-) is the monochloroacetate CHzClC00HOH + concentration, and the asterisk refers to labeled CH2OHC00Hf C1- (A) species. If the exchange reaction is first order with respect to each of the exchanging species, eq. 6 becomes, upon integration
+
(Cl-)t
+
&,(t)dt =
L-
(MCA-)t
=
+
U
+ p(t)
b - p(t)
(2)
(3)
For the exchange reaction CH2CICOO-
+ C1-*
-4
CHzCl*COO-
+ C1-
(B)
In (1 - F,)
R,(t)
=
Ic,(Cl--)"(MCA-)"
(4)
and (1) Presontedbefore the division of Physical Chemistryat the March, 1962 National Meeting of the American Chemical Society in Washington, D. C.
- kX(a+ b)t
(7) As reaction A progresses, some undissociated chloroacetic acid is formed by association of the protons with chloroacetate ions. A kinetic study of the system then would be complicated by the change in concentration of the chloroacetate ion and by the reactions CH2ClCOOH
the rate is
=
+ HOH CHZOHCOOH
+ H- + CI-
(C)
and ( 2 ) C. P. Luehr, G,E. Challenger, and B. a. Masters, J . Am. Chem. Soc., 78, 1314 (1956). (3) R. A . Kenney and F. J. Johnston, J . Phys. Chem., 63, 1426 (1 969).
